The present invention relates to a method of producing a polishing sheet material suitable for use in polishing and finishing the surface of a semiconductor IC wafer, glass, metal and the like.
A conventional method of producing the polishing sheet material or cloth comprises impregnating a nonwoven fabric with a polyurethane resin solution, dipping the impregnated fabric in a water bath or an aqueous solution of dimethylformamide so as to effect complete coagulation, washing the product with water, and drying the washed product. In the sheet material produced by the conventional method, the polyurethane resin has a porous structure, so that it is too soft, thereby causing the edges thereof to be "sagged" and suffer from flagging or sugging at the time of use, and so it has been impossible to obtain a wafer having sharp edges. Accordingly, an attempt has been made in order to strengthen the fiber network structure, which includes the steps of slicing the sheet material obtained by the above-described conventional method, removing the skin layer from each surface thereof, and performing high-temperature heat treatment so as to melt the polyurethane resin and to effect fusion around the fibers. Another attempt was made to use polyurethane resins having different melting temperatures to facilitate the above-described melting and fusion (see Japanese Patent Laid-Open No. 62-297061/1987).
The present inventor conducted a study to obtain an improved polishing sheet material, and proposed a method of obtaining the polishing sheet material which comprises impregnating a nonwoven fabric of a synthetic fiber having a network structure with a polyurethane solution, performing incomplete coagulation of the same, removing any remaining solvent in an atmosphere heated at 75.degree..+-.5.degree. C., and performing complete coagulation, as disclosed in Japanese Patent Application No. 2-7921, filed Jan. 17, 1990.
Experiments have been repeated in accordance with the method described in the above-described Japanese Patent Laid-Open No. 62-297061/1987, and observations have been made by means of scanning electron photo-micrographs. As a result, it has been found that there are a large number of residual microvoid structures in the resin portion present between fibers forming the skin layer portion of the material. Regarding the attempt to eliminate the microvoids by subjecting the same to high-temperature heat treatment so as to effect the melting and fusion of the resin, it had a limitation of its own attributed to the narrowness of the temperature control range for the high-temperature heat treatment. On the other hand, a considerable improvement has been achieved by the invention disclosed in the above-described Japanese Application No. 2-7921. However, in respect of the prolongation of the life of the polishing sheet material which is an especially strong desire at present, it is unsatisfactory as yet.